check = ['word']
for i in range(len(w_table)):
    if w_table.iloc[i, 4] != 'Null':
        if w_table.iloc[i, 4] in check:
            w_table = w_table.drop(w_table.index[i])
        else:
            check = check.append(w_table.iloc[i, 4])
        w_table.index = np.arange(len(w_table))


> TypeError                                 Traceback (most recent call
> last) <ipython-input-74-40b9156195fa> in <module>()
>       2 for i in range(len(w_table)):
>       3     if w_table.iloc[i, 4] != 'Null':
> ----> 4         if w_table.iloc[i, 4] in check:
>       5             w_table = w_table.drop(w_table.index[i])
>       6         else:
>
> TypeError: argument of type 'NoneType' is not iterable